It is known that a conventional electric power supply system can perform self-sustained operation even when the commercial power supply stops. For example, in patent literature 1, a system includes first and second electricity storage systems, and uses one of the electricity storage systems for charging, and the other electricity storage system for discharging.
Patent Literature 1: JP 2013-162686A
In the patent literature 1, because one of the electricity storage systems is used for charging and the other electricity storage system is used for discharging, the flexibility in charging and discharging is low. In addition, when the remaining charge amount of one of the electricity storage systems becomes equal to or less than a predetermined value, the electricity storage system for charging and the electricity storage system for discharging are exchanged. Consequently, there is the possibility that electric power is shifted uselessly from one of the electricity storage systems to the other electricity storage system.